1. Field of the Invention
The present invention relates to a housing structure for accommodating electronics apparatus or circuit.
2. Description of the Related Arts
Recently, electronics apparatus have often been installed in an environment in which it can easily pick up an electrostatic charge. For example, service personnel may become electrostatically charged when they walk on a carpet covering the office floor to approach the electronics apparatus. Then an electrostatic discharge occurs between the human body and the apparatus to be attended when the service personnel touches the latter, which results in the erroneous operation of the apparatus and/or a system including the same.
Accordingly, there has long been an eager desire for a housing for accommodating the electronics apparatus, providing less probability of erroneous operation of the apparatus therein and/or a system including the same, even though the charged human body touches the housing; in other words, a housing ensuring the stable operation of the electronics apparatus accommodated therein.
One example of prior art housings is illustrated in FIGS. 5 and 6. In general, the housing 20 is made of highly electro-conductive material such as metal, typically a steel plate, and frame-grounded (hereinafter referred to as "FG"). It encircles the electronics apparatus to protect the same from damages caused by being subject to an electrostatic discharge (hereinafter referred to as "ESD"). The housing also serves for mitigating electromagnetic interference (hereinafter referred to as "EMI") and for securing the safety of the contained apparatus. In this connection, a printed circuit board 4 constituting the electronics apparatus is suitably signal-grounded (hereinafter referred to as "SG").
EMI is defined by the International Electric Standard Conference as "EMI is phenomena in which a reception of a desired electromagnetic signal is disturbed by unnecessary electromagnetic signals or noises". The ground system of an electronics apparatus is usually divided into two subsystems; the aforesaid FG and SG for determining a zero potential level of the electronics circuit; in practice, both the subsystems being interconnected with each other at a certain point and grounded together.
The process of electrostatic discharge to such metallic housing will be explained by the drawings as follows:
FIG. 6(c) is an equivalent circuit for the explanation of ESD, in which the lefthand side from a dotted line shows a human body replaced by a condenser C.sub.1 and a resistor R.sub.1 connected in series and the righthand side shows a metallic housing directly frame-grounded.
Now assume that the voltage charged in the condenser C.sub.1 is 10 kV due to the electrostatic charge of human body and the resister R.sub.1 has a value of 150 .OMEGA.. ESD occurs when the human body touches the metallic housing accompanying a large electric current flowing to FG. Such electric current has a maximum value, for example, of 67 A which is decreased by half after 10 ns, as shown in FIG. 6(a).
In the metallic housing 20, a printed circuit board 4 on which electronic circuits are formed is accommodated and connected to SG, as shown in FIG. 5.
As stated above, a larger electric current flows in the housing when ESD has occurred, which emits a powerful electromagnetic wave as well as creates a potential difference between points a and b on the metallic housing, as shown in FIG. 6(b), because the impedance thereof is not zero. This potential difference causes the FG potential level to fluctuate. Accordingly, the electronics circuits formed on the printed circuit board within the housing are affected by the electromagnetic wave thus emitted and further, when FG and SG are connected with each other, the zero level of the printed circuit board fluctuates due to the variation of FG potential level, which of course increases the probability of erroneous operation of the circuits.
In this connection, the definition of erroneous operation includes the following, which are different due to the functions of the respective apparatuses:
1. Damage of a CPU itself, or misoperation of a CPU out of the programmed routine, and noise on a display, in the case of an information apparatus.
2. Deterioration of transmitted signals or mistransmission of signals in the case of a communication apparatus.
3. Damage of electronic devices in the case of a domestic electric apparatus.
Particularly, in a communication apparatus having a high speed digital transmission function, an erroneous operation is defined by a bit error and electronics parts used therein become sensitive and easily affected by ESD because they are small-sized and of a lower energy comsumption type.
To prevent such ESD troubles, the following countermeasures are taken in the prior art.
1. The impedance of the ground system of the electronics apparatus including SG and FG is lowered to decrease the abovesaid fluctuation of the FD potential.
2. The electromagnetic wave generated by ESD is shielded to mitigate its influence on the electronic circuits.
3. Electronic parts and circuit structures having excellent noise durability are selected so that they can normally operate without error even though the noise occurs as a result of the fluctuation of FG potential due to ESD.
These countermeasures, however, are not satisfactorily effective even when all of them are adopted in combination, for the high speed digital communication apparatus which is evaluated by a severe test standard, for example, no bit error is permitted for 10 kV ESD.